This invention relates to a thermoplastic lid for thermoplastic containers. More particularly, the invention relates to a thermoplastic lid which is intended to be used as a closure for hot filled products.
It is known from U.K. Specification No. 1,367,338 to make cupped articles from a sheet of a thermoplastic material such as polypropylene. Such articles can also be made from billets of the same materials. The method comprises introducing a thin sheet or a billet of the thermoplastic material in the solid phase state, i.e. below the crystalline melting point, between a forming plug and a hollow mold having an end opening, moving the forming plug into the mold opening with the sheet to carry the stretched portion of the sheet into the mold, and introducing a pressure fluid into the stretched portion of the sheet to form the article in the mold. The above method is particularly suitable for making cupped articles of polypropylene and has met with considerable commercial success.
It is known that after filling and sealing a yieldable body such as a thermoplastic container made from such a cupped article, there is a tendency for the sidewalls thereof to deform or panel inwardly under certain conditions. This deformation of the sidewalls results from the pressure differential between the inside and the outside of the container and these deviations may be brought about by various physical or chemical conditions. In the packaging of food, various materials including fluids such as juices, syrups, salad oils and the like are oftentimes brought to elevated temperatures before introduction into thermoplastic containers. It has been found that there is a marked tendency for such containers to distort inwardly as described above. For example, when hot-filled containers are allowed to cool, the internal pressure will gradually decrease whereby the external atmospheric pressure causes their sidewalls to indent, panel or otherwise partially collapse. This condition renders the containers unacceptable to the ultimate consumer.
Aside from the problems associated with such hot-filled containers, there are other related packaging situations where chemical reactions cause noticeable reductions in the pressure differentials of the container resulting in the paneling or deformation of the sidewalls. For example, when lubrication or motor oil is packaged in a plastic container and sealed, chemical reactions take place between the various hydrocarbon constituents and any residual oxygen, e.g. air, causing the total pressure within the container to decrease. With this drop in pressure, there results an inward paneling of the sidewalls in order to equalize or compensate for the decrease in internal pressure. Here again, as with the hot-filled container, the containers are unacceptable to the ultimate consumer.
Aside from an undesirable appearance, the container itself loses column strength and sidewall symmetry which presents a problem in stacking them for storage, display and the like. Since the reduction in pressure cannot always be practically avoided, the present invention provides a container configuration wherein a portion of the lid of the container compensates or yields as more fully disclosed hereinafter in preference to the sidewalls of the container.